The size of your target area can definitely have an impact on the size of your groups....it is part physical and part mental. Physical in that you don't have as small of an aiming point to focus on. Mental in that most archers became a bit lax in their focus when a larger target is presented.

I have much the same problem when it comes to certain color target faces. I can shoot great if it is two colors...a light and a dark one. Throw in three colors and make them all light in hue and I can't group worth a darn.